The present invention relates to remote manipulators, and more particularly, to a vision system for sensing and verifying the end effector position of a robotic device for nuclear steam generator inspection and repair.
Nuclear steam generator servicing is usually performed remotely, using robotic cantilever devices commonly referred to as manipulators. A typical nuclear steam generator has thousands of tubes, affixed at their ends to a tube sheet. A preliminary step in servicing a tube with a remotely operated manipulator is to position the tooling component of the manipulator, typically referred to as the end effector, under a preselected tube end opening.
Positioning errors can have serious consequences. A steam generator tube that has been selected for inspection or repair could be bypassed because of a positioning error, and a neighboring tube inspected, sleeved, cut or plugged instead. The most serious ramification of positioning error is not the possibility that a satisfactory tube is serviced, but rather, that a tube selected for a service does not receive such planned service. If the missed tube has a defect of significant magnitude, a leak could occur during the subsequent operation of the steam generator and result in an unplanned shutdown to make the correct repair. This background highlights the need for both a reliable positioning system for remote manipulators, and for accurate independent position verification techniques.
Currently, a variety of schemes are used for positioning manipulators. The more advanced techniques rely on joint sensor feedback. Through robotic techniques known as kinematics and inverse kinematics, the joint angles required to reach a point on the tube sheet are calculated, and positioning is carried out in conjunction with inverse dynamics calculations, using joint sensor feedback. A less sophisticated method requires temporary placement of mechanical templates on the steam generator tube sheet, which identify the tube locations with alphanumeric symbols. The manipulator operator can then position the end effector by video feedback on a monitor. More recently, a tube stamping mechanism has been developed, which labels each tube permanently, allowing operators to position tools with video feedback.
All of the above mentioned techniques have some drawbacks. The manipulators which rely on sensors experience spurious movement (usually drooping) because of overweight problems associated with the resolvers. These robotic devices are also more expensive to deploy than their alternatives and are subject to potential calibration errors. The mechanical template approach results in increased radiation exposure for the personnel responsible for the template installations and deinstallations and is subject to template installation errors. The tube stamping method is currently performed only when a steam generator is relatively clean and where tube spacing is relatively large.